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J76-26129 (NASA-!; ,0 -14}? 17 ti ) T»iT l't )ISCIPLIhAi>Y g S i1i COAPAkATIVF PLANETUL66Y Ir4VF^Ti t AT 141 ANNUAL :iTATUS kt.P ts'T, 1 JtPI. 1975 - 3J JtJL. 19 P Hf, g 3.^J CSl`.L J33 G3/91 UNC(.AS 1971 (C kRNELL 11NIV.) 42e37

CORNELL UNIVERSITY Center foa^ 1Zadiophysics and S'pace Research

TT,, ITHACA, N. YY

CRSR 636 ANNUAL STATUS REPORT to the

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION under

NASA Grant NGR 33-010-220

INTERDISCIPLINARY INVESTIGATIONS ^O P COMPARATIVE PLANETOLOGY

July 1, 1 ,975 through June 30, 1976 ^^212a2^-,

JUN 1976 Principal Investigator: Prof. Carl Sagan RECE'Vca

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TABLE OF CONTENTS

A. Martian Channels ...... 1 B. Variable Features on Mars ...... 2 C. The Surfaces and Origins of the Martian Satellites ...... 7 D. The Surface of Venus ...... 11 E. Miscellaneous . . . . • ...... 14 k

The following are abstracts q f paper., published within the last vear (or in press) on scientific results supported wholly or in part by the Planetolof,y Programs Office, NASA Headquarters and written by personnel of the Laboratory for

Planetary Studies:

A. MARTIAN CHANNELS

(1) Distribution of Small Channels on the Martian Surface. Icarus 27, 25-50 (1976). The distribution of small channels on Mars has been mapped from Mariner 9 images, at thA 1:5 000 000 scale, by the author. The small channels referred to here are small valleys ranging in width from the resolution limit

of the Mariner 9 wide-angle images (ti 1 km) to about 10 km. The greatest density of small channels occurs in dark cratered terrain. This dark zone forms a broad sub- equatorial band around the planet. The observed distribution may be the result of decreased small-channel visibility in bright areas due to obscuration by a high-albedo dust or sediment mantle. Crater densities within two small- channel segments show crater size-frequency distributions consistent with those of the oldest of the heavily cratered plains units. Such crater densities coupled with the almost exclusive occurrence of small channels in old cratered terrain and the generally degraded appearance

of small channels in the high-resolution images (ti 100 m) imply a major episode of small-channel formation early in Martian geologic history.

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(2) A Comment on the Paper by Florenski et al., Icaruo

26, 230 (1975). The nature of Nirgal Vallis may compromise the findings of Florenski et al.; comprehensive image references would

enhance the analysis.

B. VARIABLE FEATURES ON MARS

(3) A Numerical Circulation Model with Topography for the Martian Southern Hemisphere. Journal of the Atmospheric Sciences, in press. A quasi-geostrophic numerical model, including friction, radiation, and the observed planetary topography, is applied to the general circulation of the Martian atmosphere in the

southern hemisphere at latitudes south of about 35 0 . Near equilibrium weather systems developed after about 5 model days. To avoid violating the quasi-geostrophic approximation, only 0.8 of the already smoothed relief was employed. Weather systems and velocity fields are strikingly tied to topography. A 2mb middle latitude ,jet stream is found of remarkably terrestrial aspect. Highest surface velocities, both horizontal and vertical, are predicted in western Hellas Planitia and eastern Argyre Planitia, which are common sites of origin of major Martian dust storms. Mean horizontal

velocities > 3 0 m/s and mean vertical velocities < 0.2 m/s are found ,just above the surface velocity boundary layer, apart from eddy velocity contributions. When consideration

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is taken of scaling to full topography and the probable gustiness of Martian winds, it seems very likely that the general circulation is adequate, at certain times and

places, to transport dust from the surface of Mars, as observed. Certain sources and sinks of vertical dust streaming are suggested; the entire south circumpolar zone appears to be a dust F31nx in winter.

(4) Fluid Transport on Earth and Aeolian Transport on Mars. Icarus 26, 209-218 (1975). Experimental data on cohesion-free particle transport in fluid beds are applied, via a universal scaling relation, to atmospheric transport of fine grains on Mars. It may be that cohesion due to impact vitrification, vacuum sinter- ing, and adsorbed thin films of water are absent on Mars-- in which case the curve of threshold velocity versus grain size may show no turnup to small particle size, and one micron diameter grains may be injected directly by saltation into the Martian atmosphere more readily than 100 micron diameter grains. Curves for threshold and terminal velocities are presented for the full range of Martian pressures and temperatures. Suspension of fine grains is significantly easier at low temperatures and high pressures; late after- noon brightenings of many areas of Mars, and the generation of dust storms in such deep basins as Hellas, may be due to this effect.

I i ` I }f (5) Physical Properties of Suspended Aero:.olo from their Infrared Spectra: Application to the Martian Duct Storm of 1971. Icarus, submitted for publication. Infrared spectra of Mars obtained from the Mariner 9 spacecraft during the Martian dust storm of 1971 were analyzed to determine the chemical composition and size distribution of the suspended dust grains. The infrared optical constants of granite and mont-morillonite were measured for the analysis which employs a new, approximate, technique for calculating the emergent intensity from a scattering, emitting dust cloud. The results show that the dust was composed of an intermediate or acidic igneous rock or a weathering product such as a clay. The clay mineral montmorillonite was not the only component of the dust, but the results do not exclude (or require) montmorillonite as a component of a possible mixture. The dust could not have been composed of a basic igneous rock like basalt, nor could basalt have played a dominant role as a component of a mixture. The mean dust particle radius was a few microns, the dust size distribution closely resembled terrestrial dust size distributions in regions remote from a dust source, and the Martian dust size distribution did not change significantly during the dust storm dissipation. The composition and size distribution of Martian dust have a number of geological and meteorologic implications for Mars. We have also studied the way optical

l _ properties of dust interact to create infrared spectra. The transmission spectrum maximum (Christiansen frequency)

is not compositionally si gnificant for dust particle:, in the size distributions we studied. Many optical properties interact to create transmission spectra of dust powders and we show that powder transmission spectra cannot be used to obtain infrared optical constants.

(6) Variable Features on Mars.V: Evidence for Crater

Streaks Produced by Wind Erosion. Icarus 25, 595-601 (1975). Evidence is presented to show that the ragged dark

streaks which appeared behind many Martian craters severa' months after the end of the 1971 g lobal duststorm were produced by wind erosion of a thin surface veneer of duststorm fallout.

(7) Variable Features on Mars. VI. An Unusual Crater Streak in Mesogaea. I carus 27, 241-253 (1976). An unusual, prominent dark streak located in Mesogaea (near 8 0 N, 191 °W) is described. Its appearance is unlike that of most dark streaks on Mars, many of which have ragged outlines, are variable on short time-scales, and are pre- sumed to be erosional. The Mesogaea streak has a tapered,

smooth outline, and no changes within it were observed. We suggest that this streak is depositional and that the

low-albedo material originated within the associated crater itself. The source area is identified with a compact,

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low-albedo region on the crater floor. Two possible origins for the dark material are suggested: (1) deflation from a recently exposed, relatively unconsolidated subsurface deposit, and (2) production of ash by a volcanic vent.

(8) Variable Features on Mars. VII. Dark Filamentary Mar:..ngs on Mars. Icarus, in press. Some Mariner 9 B-frames show networks of Criss-crossing rectilinear albedo markings typically 10 km long by 100 meters wide. This paper discusses the location, variability and possible nature of these dark filamentary markings. Although not common on Mars, the markings are concentrated in at least two areas: Depressio Hellespontica and Cerberus/ Trivium Charontis. For Depressio Hellesponviea their emergence coincided with a general darkening of the region which occurred 3 months after the end of the 1971 global duststorm. The very definite crass-cross pattern of many of these markings suggests that they may be controlled by ,joints. It is unlikely that the markings are linear dunes.

(9) Variable Features on Mars. VIII. Pandorae Fretum. Icarus, to be submitted. During the Mariner 9 mission a region near (343°W, 24 0 S) was monitored for changes over a period of 5 months (25 Jan to 30 June, 1972). This region lies within the boundaries of the classical albedo feature Pandorae Fretum whose changes are well-documented at low resolution by

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Earth-based photography. The Mariner 9 picture provide:, a record of the changes at aul'ficiently high resolution that the "_.,ely mechanism, aeolian redistribution of surface materials, can be inferred.

C. THE SURFACES AND ORIGINS OF THE MARTIAN SATELLITES

(10) Photometric Functions of Phobos and Deimos. I. Disc- Integrated Photometry. Icarus, in press. We have used the integrated brightnesses from Mariner 9 high-resolution photographs to determine the large phase angle (20 1 to 80 0 ) phase curves of Phobos. and Deimos. The derived phase coefficients are 0 = 0.032 + 0.001 mag/deg for Phobos and 0 = 0.030 + 0.001 mag/deg for Deimos, while the corresponding phase integrals are gPhobos - 0.52 + 0.03

and gDeimos = 0.57 + 0.03. The predicted intrinsic phase

coefficients of the surface material are (3 i = 0.019 mag/deg

and ^ i = 0.017 mag/deg for Phobos and Deimos, respectively.

The phase curves, phase coefficients ai,u piiase integrals are typical of objects whose surface layers are dark and intricate in texture, and are consistent with the presence of a regolith on bosh satellites. The relative reflectance of Deimos to Phobos is 1.15 + 0.10. The presence of several bright patches on Deimos could account for this slight difference in average reflectance.

(11) Photometric Funetione, of Phobos and Deimos. II. Surface Photometry o;' Deimo;i. Icarus, in press. To a good approximation the face of Deimos observed 1 8 { by Mariner 9 is covered uniformly by a dark, texturally complex material obeying a liapke-Irvine scattering law.

The intrinsic 20 0 to 80 0 phase coefficient of this material is 01 = 0.017 + 0.001 mag/deg, corresponding to a dioc- integrated value of 0 = 0.030 mat;/deg. There is also evidence of a slightly brighter (by ti 30%) unit near come craters, which may have been produced by the cratering events. Its texture appears to be identical to that of the average material. No evidence of quasi-specular reflection has been found, suggesting that large-scale exposures of unpulverized rock are absent.

(12) Photometric Functions ov Phobos and Deimos. III. Surface Photometry of Phobos. Icarus, in press. At least three large areas on the surface of Phobos are covered by a dark material of complex texture which scatters light according to the Hapke-Irvine Law. The average intrinsic and disc-integrated phase coefficients of this surface material are $ J = 0.020 + 0.001 mag/deg and 0 = 0.033 mag/deg, respectively. These values are slightly greater than the values found for Deimos in Paper II. On the largest scale the surface of Phobos is rougher than the surface of Deimos, perhaps accounting for the slightly greater phase coefficients. Contrary to the situation on Deimos, no definite regions of intrin- sically brighter material are apparent on Phobos. This difference could account for the slightly lower average

a, n ..,------..,.e.P.._,s jam:

reflectance of Phobos relative to Deimos. No evidence for large exposures of solid rock ha^i been found in the three areas studied.

(13) Predicted Lighteurves of Phobos. and Deimos. Icarus, in press. Using Mariner 9 result:" on the shapes, rotation periods and photometric functions of Phobos and Deimos we calculate approximate orbital lightcurves for the two Martian satellites. The prediction is that both Phobos and Deimos should show orbital brigh'ness fluctuations detectable from Earth. For Phobos the detectable amplitude is predicted to be about 0.1 mag; for Deimos, 0.2 mag.

(lh) Mars: Satellite Origin and Angular Momentum.

Icarus 25, 588-59 11 (1975). The origin of Phobos and Deimos is considered with a view to accounting for the existence of very small satellites with circular orbits in the Martian equatorial plane, and simultaneously for the suspected angular momentum deficiency of the Mars system. All models considered failed to satisfy at least one requirement, and the problem is considered more puzzling than is at first apparent. The Martian angular momentum deficiency, if physically significant, may be unrelated to the present satellite,' origin, but might relate to a large ancient satellite, long ago destroyed. Accretion onto Mars of large amounts of asteroidal dust Lrought in by Poynting-Robertson drag t

1')

may have some bearing on the angular momentum

problem.

(15) The Equilib.,ium Figuroo. of Phobos and Ot)- , ^ Small Bodies. Icarus, in press. The shape of a close planetary satellite is distorted from a self-gravitating sphere into a triaxial ellipsoid maintained by tidal and centrifugal forces. Using the family of Roche ellipsoids calculated by Chandrasekhar, it should be possible in some cases to determine the density of an inner satellite by an accurate measurement of its shape alone. The equilibrium figure of Phobos is expected to be the most extreme of any satellite. The shape of Phobos as observed by Mariner 9 approaches but appears

not to be a Roche ellipscid, although the uncertainties of measurement remain too large to exclude the possibility. In any case, Phobos is so small that even the low mechanical strength of an impact-compressed regolith is sufficient to maintain substantial departures from ehe equipotential figure. If larger close satellites, particularly Amalthea, are found to be Roche ellipsoids, their densities can be I estimated immediately from the data presented. Asteroids of size comparable to Phobos and Deimos appear to have more irregular shapes than the Martian satellites. This may reflect the absence of a deep regolith on those asteroids due to the low effective velocity for impact ejects. For Phobos and Deimos, on the other hand, r 11

ejecta will tend to remain in orbit about Maro until

swept up again by the satellite, contributing to a deeper

equilibrium layer of debris.

D. THE SURFACE OF VENUS (16) Erosion and the Rocks of Venus. Nature, in press. The remarkable photographs of the surface of Venus returned by the Venera 9 and 10 spacecraft have revealed the presence, in two different sites, of a variety of rocklike forms, some angular and some smooth. Press re- ports express surprise at the absence of very efficient erosional mechanisms. It may be useful to point out that, instead, it is the presence, riot the absence, of erosional mechanisms on Venus which is surprising. The degree of erosion of surface rocks on Venus will, of course, be determined by an equilibrium between the rate of production and the rate of destruction of surface rocks. The principal causes of erosion of terrestrial roc's -- running water; diurnal and seasonal temperature changes, particularly in deserts; and aeolian abrasion -- are all absent on Venus: The surface temperature of 750 K is above the critical point temperature of water. Ground-based radioastronomical measurements; a comparison of the temperatures measured by Venera spacecraft at a variety of solar zenith angles; and the high heat capacity of the massive Venus atmosphere together clearly demonstrate that the diurnal temperature differences are a few degrees K at most. The obliquity

l I la of the rotation axis of Venus is so small that there are effectively no seasons on the planet. The efficiency of aeolian abrasion depends on the velocity to a power > 3; since both theory and observation show the velocities in the .lower atmosphere of Venus to be about an order of magnitude less than at comparable regions in the Earth's atmosphere, it follows that sandblasting on Venus is at most 10 -3 as efficient as on Earth. The problem is to find a suitable source of erosion of surface rocks -- a problem somewhat similar to that caused by the radar discovery of large, presumably impact, basins which, in comparison to their lunar, martian and mercurian equivalents, are remarkably shallow. Two mechani s ms for the erosion of crater ramparts on the surface of Venus can be suggested; I propose that they also may be important fur erosion of the rocks photographed by Venera 9 and 10. The atmosphere of Venus contains hydrochloric acid at mixing ratio of about 10 -6 ; hydrofluoric acid at mixing ratio of about 10 -a ; and sulfuric acid at larger mixing ratios which, however, are as yet undetermined for the lower atmosphere of the planet. Chemical weathering; by such a mixture of strong acids, even in very dilute concentrations, may, over sizeable periods of time, be quire adequate to erode angular projections of siliceous rocks.

A second possibility arises from the high surface temperature of Venus. While these temperatures are not 5

so high as to melt silicates, they are high enough to bring many materials of not exceptional geochemical rarity near or to their melting points -- NaOH, KOH, HgS and KNO2 to mention a few. If the rocks of Venus are comprised of such materials even in abundances of a few tenths of a percent, the rheo7c6ical properties of their ].ow melting point components may, over long periods of time, be adequate to soften the contours of surface rocks. Deforma- tion of the walls of large basins will occur over long periods even if the lowest melting point of abundant con- stituents is considerably above 750 K. With typical terrestrial values of the subsurface temperature gradient, the high surface temperature of Venus implies that the melting points of silicates should be reached a few tens of km subsurface. The "granitic" values of the uranium/potassium/thorium radioisotope ratios, as determined by Venera 8, suggests that a terrestrial value to the subsurface temperature gradient may be a good first approximation. In this case, access of magmatic material from the interior of Venus to its surface should be considerably easier than on Earth and significant fractions of the surface may be frozen lava fields having reached thermal equilibrium at the low temperature of 750 K. This may provide both a source of rocks too young to have yet been eroded and a means of filling large craters and impact basins. 1 f{

Venera 9 and 10 are the first spacecraft to obtain in situ photographs of the surface of another planet. The remarkable photographs which they have obtained open a new field of small scale planetary geology.

E. MISCELLANEOUS (17) The Canals of Mars: An Assessment after Mariner 9. Icarus 25, 602-612 (1975). The Lowellian canal network has been compared with the results of Mariner 9 photography of Mars. A small number of canals may correspond to rift valleys, ridge systems, crater chains, and linear surface albedo markings. But the vast bulk of classical canals correspond neither to topographic nor to albedo feature:, and appear to have no relation to the real Martian surface.

(18) Kepler and Mariner 9. Vistas in Astronomy, Vol. 18, pp . 905-908, Oxford: Pergamon Press (1975)• The historical connection of the speculations of Johannes Kepler about martian moons with the exploration of these moons by Mariner 9 is outlined.

(19) On Solar System Nomenclature. 1Qarus, in press. Arguments are presented for naming topographic features on other solar system objects after human beings other than astronomers; and to institute a more consistent scheme for Jovian satellite nomenclature.

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During the course of the grant year a wide range of other work has been accomplished which is not yet in press, and which is more fully described in trio body of the renewal proposal. Included are some important advances in the general theory of fluvial channels on Earth and Mars; the physics of ice-choked rivers on Mars; substantial progress in the development of a computer accessed catalog of martian channel and micro-channel characteristics important for any comprehensive understanding of channel physics and climatic change on Mars; martian climatic models; an analysis of the diversity of planetary systems which can be generated by computer simulations of gravitational accretion under a variety of initial conditions; near completion of the reduction of the Arecibo 1973 delay doppler radar mapping of Mars and its cross correlation with Mariner 9 photography; significant progress in the development of simple deductive theories of sand ripj..es on Earth and Mars; discussion of the origin of sand and dust storms on Mars with particular attention to the Hellespontica/Hellas site; and ar. overall pre-Viking review of windblown dust on Mars. Within the past year the principal investigator served as Chairman of the Division for Planetary Sciences of the American Astronomical Society; as Chairman of the Astronomy Section of the American Association for the Advancement of Science; and has recently been appointed a member of the Physical Sciences Committee, SPAC, NASA Headquarters. He served as Jacob Bronowski Lecturer at the University of

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Toronto, as Royal Institution Lecturer at the Royal Institution in London and as Danz Lecturer at the University of Washington. He also received three honorary Doctor of Science degrees, from Rensselaer Polytechnic Institute, Denison University, and Skidmore College. Among his popular works published which relate to the subject matter of this grant were British, German, French, Spanish, Dutch and Italian edit!.ons of The Cosmic Connection: An Extraterrestrial Perspective, and a British edition of Other Worlds; as well as "The Solar System" (Scientific American, Vol. 233, pp. 22-31) and "The Planets" in Man and Cosmos: The Guggenheim Lectures (New York, W. W.

Norton), pp. 68-100.